MPEG2 Recording Quality Question

[wayner]

I am currently using MPEG2 Max Quality (5.9GB/hr) as the default recording quality for my PVR-150 tuner. The source for this tuner is an SD digital cable box. Is this quality level overkill? In other words, what is the bitrate of SD digital cable and I guess there isn't any point in using a higher setting, is there? It's kind of like converting a 128kbps MP3 file to analog and then reencoding to digital - there is no point in using a bitrate higher than 128 kbps, is there?

If I remember correctly a full ATSC HD feed uses a bitrate of 19 Mbps. This equates to about 8.5GB/hr. I thought that SD used about 1/6 the bitrate of HD - that would then imply using the Sage Good quality setting at around 1.4GB/hr.

(Here is a post on AVS showing that FIOS SD bitrates range from about 2.5-5 Mbps for SD specialty channels. This would imply that SD uses more like 1/5 the bitrate of HD a little more than I thought.)

[gplasky]

My opinion, Max Quality MEPG2 for an SD show is definitely overkill. With the HD-PVR my 1 hour TV shows, (Law & Order or Royal Pains) is around 3.7 GB for the 1 hour show. With the HDHR, Fringe ends up being about 6.7 GB. When I recorded analog I recorded at Standard DVD quality so I could burn them easily. Those were 3-3.5 GB for a 1 hour show.

Gerry

[paulbeers]

I'm not sure where you came up with the 1/6th of HD. Depending on whether 720P or 1080i, you are looking at about 16-20mbps for HD. DVD quality is generally considered 3.2GB/hr (by mpeg2 encoders) which equates to (3.2gb/hr x 1024 x 8 / 3600) 7.2 mbps. This means you are only looking at about 1/2 to 1/3 of the same bit rate. As Gerry stated, max bit rate encoding is completely overkill. I currently use 3.2GB/hr only because I have the space. For years I used 2.0GB/hr and was completely happy with it.

[stanger89]

I always found 2-3GB/hr to be the sweet spot for SD.

[wayner]

Originally Posted by paulbeers

I'm not sure where you came up with the 1/6th of HD. Depending on whether 720P or 1080i, you are looking at about 16-20mbps for HD.

Isn't SD content 480 lines of resolution. 4/3 * 480 = 640 so for SD you have 307,200 pixels on the screen. For 720p the resolution is 720x1280 which is 921,600 pixels or three times the number of pixels. At 1080 you have resolution of 1080x1920 or 2,073,600 pixels or 6.75X as many pixels.

So if we compare 1080i to 480i then there is 6X as many pixels. And I believe that HD has a higher colour depth than SD - so that is probably where the 6X ratio comes from, at least in terms of theory.

[doc]

I always used 'best' on the PVR cards. I think thats 3GB/hr.
I tried 'great' which is 2GB/hr but I wasn't happy with that.

Digital SD in the UK uses about 1GB/hr
HD in the UK uses about 5GB/hr

[stanger89]

Originally Posted by wayner

Isn't SD content 480 lines of resolution. 4/3 * 480 = 640 so for SD you have 307,200 pixels on the screen. For 720p the resolution is 720x1280 which is 921,600 pixels or three times the number of pixels. At 1080 you have resolution of 1080x1920 or 2,073,600 pixels or 6.75X as many pixels.

HD is 6x the resolution, but compression doesn't exactly scale linearly.

So if we compare 1080i to 480i then there is 6X as many pixels. And I believe that HD has a higher colour depth than SD - so that is probably where the 6X ratio comes from, at least in terms of theory.

They're both 8-bit.

[reeven]

I use analog cable on analog leadtek tuner.

With 2gb/hour ( Great Quality) image is perfect but sound is poor on that.
With 3,2gb/hour ( Standard Quality) image is perfect but has poor sound also.
I wish to have an custom quality, with 3,2/hour from standard but with sound from max quality.

[Tiki]

Originally Posted by wayner

I am currently using MPEG2 Max Quality (5.9GB/hr) as the default recording quality for my PVR-150 tuner. The source for this tuner is an SD digital cable box. Is this quality level overkill? In other words, what is the bitrate of SD digital cable and I guess there isn't any point in using a higher setting, is there? It's kind of like converting a 128kbps MP3 file to analog and then reencoding to digital - there is no point in using a bitrate higher than 128 kbps, is there?

If I remember correctly a full ATSC HD feed uses a bitrate of 19 Mbps. This equates to about 8.5GB/hr. I thought that SD used about 1/6 the bitrate of HD - that would then imply using the Sage Good quality setting at around 1.4GB/hr.

(Here is a post on AVS showing that FIOS SD bitrates range from about 2.5-5 Mbps for SD specialty channels. This would imply that SD uses more like 1/5 the bitrate of HD a little more than I thought.)

I use a couple of PVR-150's to record from cable boxes using the composite (single RCA jack) connection (my boxes don't have S-Video out). I have the best results using the "DVD Standard Play" setting. This creates MPEG-2 files that are 3.2GB per hour.

I also have an HDHomerun that creates similar quality MPEG-2 files that are about 1/2 the size. With the HDHomerun, there is no choice for the quality, because it is grabbing the raw digital data that is being broadcast by the cable company over clear QAM. The shows were already stored in MPEG-2 format using a high quality encoder before they were broadcast. That is why they can get a very clear picture with a smaller file size.

The thing to remember when using an analog capture card like the PVR-150 is that a lot depends on the quality of the analog signal. If you are connected to a cable box, the box is decoding the signal from the cable company (which could be analog or digital) and sending an analog signal out the RF(Coax), S-Video, and Composite connections. The PVR-150 is grabbing an analog signal from one of those sources. If that signal has a lot of "noise" (looks grainy or snowy), the MPEG encoder on the PVR-150 will have a tough time making a decent quality file unless the bitrate it uses is very high (and then the best you can hope for is a picture that looks close to the noisey original).

The reason is that digital compression relies on there being a lot of redundancy in an image or video. If one frame of video is 640x480 and the whole screen is black, there are two ways I could choose to describe it. Option1: Draw a black square that whose top left coordinates are 0,0 and whose bottom right coordinates are at 640,480. Option 2: Draw a black dot at 0,0. Draw another black dot at 0,1. Draw another black dot at 0,2... keep repeating until I've filled every pixel on the screen. Obviously Option 1 is much more efficient. If a digitial video file had to store the color and intensity of every pixel in every frame of the video, the files would be huge. Video codecs use a whole bunch of algorithms to find ways to shrink these files down by taking shortcuts to describe the picture and the changes from one frame to the next.

Something like white noise is very difficult to encode efficiently because every pixel is random and is randomly and continuously changing. So, if you have a noisey analog video, an MPEG encoder will spend as much time trying to encode all the snow as it does trying to encode the actual video and you end up with either a very large file or one with really bad quality.

So, bottom line is that the cleaner your analog source material is, the smaller you can make your MPEG files and still get good quality.

[wayner]

Originally Posted by Tiki

The thing to remember when using an analog capture card like the PVR-150 is that a lot depends on the quality of the analog signal. If you are connected to a cable box, the box is decoding the signal from the cable company (which could be analog or digital) and sending an analog signal out the RF(Coax), S-Video, and Composite connections. The PVR-150 is grabbing an analog signal from one of those sources. If that signal has a lot of "noise" (looks grainy or snowy), the MPEG encoder on the PVR-150 will have a tough time making a decent quality file unless the bitrate it uses is very high (and then the best you can hope for is a picture that looks close to the noisey original).

Thanks - useful information. The quality of my SD channels is very good - there is very little, if any noise, as my cable company (Rogers Cable) provides all channels in digital format, even those that are available in analog as well. The quality of these channels in digital is much better than the analog version of the same channel.

There is occasional quality issues with the feeds but I believe that is from the original source or at least upstream of my cable company. For example I record a lot of rugby games on Setanta sports. These games come from New Zealand, South Africa, Australia, Ireland or the UK and I am guessing that the feed is consolidated at the Setanta headquarters in Ireland. At times the compression used seems rather high that causes issues with the signal, but there is nothing I can do about that as it is likely happening as the video is being encoded for a satellite somewhere along the way.

[Savage1701]

I use PVR-USB2's that have onboard encoders and record from analog cable. I can tell a difference between the 5.9GB and 3.2GB quality levels, but I have no sound issues.

Just an FWIW.